It is known in the prior art to anchor permanently (e.g., dental prostheses or crown and bridge structures) to anchoring elements integrated into the bone (i.e., implants) through the use of extension pieces or spacers which penetrate through the mucous membrane. Such implants are most often cylindrical and screw-shaped. These implants are inserted by a separate surgical operation and are completely covered by the mucous membrane. After a minimum critical ingrowth period, a further surgical operation is effected in order to connect the implants to the actual prosthetic structure through such extension pieces of suitable length. This so called "two-step method" is utilized in order to reduce the failure frequency, which is considered too high in a one-step method. In the one-step method, the implant penetrates the mucous membrane covering the bone during the installation of the implant. The higher failure frequency in the one-step method is due to the load transferred to the sensitive interface between bone tissue and implant via the portion of the implant which is already positioned in the oral cavity above the mucous membrane during the initial period of the ingrowth phase. In addition, there is the risk of an infection in the interface between the implant and the soft tissue easily propagating along the implant wall and degrading the bone tissue before ingrowth has taken place.
However, recently controlled studies have been conducted in which good results with the one-step technique have been demonstrated, provided that the portion of the implant penetrating the mucous membrane does not project to too great of a degree beyond the surface of the mucous membrane where it will be exposed to too great of loading forces, and provided also that this portion of the implant is kept completely free from microorganisms continuously, so that infection (with accompanying destruction of the tissue) will not arise.
Provided that these conditions are satisfied, it is thus very likely that the one-step technique (also in a large scale) will provide as good of results as the two-step technique. This means that both techniques will be applied within their respective indication fields.
In the two-step method, the implant is positioned in the bone tissue such that the most superficial portion thereof will be at the same level as, or slightly above, the bone surface where ingrowth is allowed to take place. The implant becomes covered by soft tissue and remains unloaded for a period which is sufficiently long that the implant becomes intimately anchored (e.g.,"gets firmly rooted") in the surrounding bone tissue. A second surgical operation is then performed, in order to uncover the implant by punching an aperture through the covering mucous membrane (or by cutting through the membrane) and in order to connect extension pieces to the implant.
One purpose of the present invention is to provide a more rational and less expensive method, which results in an improved ingrowth into the most superficial portion of the bone and the periosteal and connective tissue located on the bone surface.
To this purpose, a bone anchoring element according to the invention is described in the appended claims.
Contrary to prior art types of implant intended for the two-step method, the implant described herein can be completely covered by the mucous membrane and can, nevertheless, be uncovered, without applying extension elements. This provides several advantages:
Reduction of costs and simplification of the manufacture through the elimination of the spacer screw (and spacer socket). PA1 The implant is suitable for use in the one-step technique, as well as in the two-step technique. PA1 The diameter of the implant can be considerably reduced, without increased risk of fracture, due to the fact that the blind hole for the fastening screw has a considerably smaller diameter than the hole for a spacer screw. This increases the indication domain considerably as far as jaws having a small buccolingual width are concerned. PA1 Edge cutting with accompanying destructive bone cutting is eliminated. PA1 Allows for a simplified secondary operation with the two-step technique, since the covering element of the implant can easily be palpated, and since the neck of the implant is easily available for direct connection to the prosthetic structure. An X-ray check, in order to certify correct connection between implant components, is eliminated, and complicating bone overgrowth is precluded. PA1 There is no need of turning up a larger flap. This increases the patient comfort, spares the jaw bone and makes the surgery simpler. PA1 The joint between the implant and the spacer is eliminated. This eliminates annoying odor, if any, and infection involving risk of bone destruction, in case the joint is located adjacent the bone edge. PA1 Optimal aesthetics, by elimination of the spacer, and by the fact that the end surface of the implant, in particularly delicate cases, can be beveled (towards the buccal side). PA1 The thickness of the soft tissue can be controlled during ingrowth by the shape of the covering element being controlled. PA1 Considerably smaller critical height of the prosthetic structure, by connection thereof directly to the implant, which facilitates the prosthetics and increases the indication domain. PA1 Minimal risk of buccal screw apertures through angled passages.
Preferably, the implant is made of a material which has a sufficient degree of biocompatibility and strength in order to be permanently implanted as an anchoring element for prostheses, crowns, or bridges. It can consist of ceramics, metal, or tissue tolerant plastics, or combinations thereof. An example of a suitable metal is pure titanium, which is prepared so that the surface thereof is completely free from organic material and which is preferably given a suitable surface roughness by turning, blasting, or by another suitable preparation technique. The implant can also be coated on the surface thereof with a material which optimizes the ingrowth of the implant to form a highly intimate connection with the surrounding bone. For example, titanium can be applied to the implant by so called plasma spraying, or hydroxyl apatite. Preferably, the implant is screw-shaped.